The present invention relates to a piezoelectric filter having a lamella-like resonator, which is provided at two opposed principal surfaces with at least one pair of electrodes each of which is connected to a connector element via electrical feeder lines. The resonator electrodes are covered by spacers; the supporting area of the spacers on the resonator surface are separated from the electrode areas. The entire filter arrangement is enveloped by a solid insulator layer.
Such a filter is known from German Pat. No. 1,591,298. There a caplike synthetic spacer is so arranged on the surface of the piezoelectric lamella that the resonator electrodes are covered. Since any contact between spacer and resonator electrode must be avoided, then, because of the small dimensions of such components, a very exact positioning between spacers and resonator lamella is required. With multi-electrode arrangements a precise positioning for each single spacer is required.
In German accepted application DAS No. b 2,459,037 a piezoelectric filter is shown which consists of a resonator disk enveloped by a shell made of a porous duroplastic synthetic-resin material in which the resonator disk has an all-around free clearance from the shell and is attached to the shell only by the electrode feeder lines developed as a hold-down component. Since, with this type of structure, the entire resonator disk freely oscillates, the oscillations are not limited to an exactly defined area of the resonator disk so that undesired spurious resonances or reverberations leading to glitches in the conducting-state characteristic cannot be avoided.
German Pat. No. 1,914,307 describes a piezoelectric filter in which the principal surface areas of the resonator lamella are provided with electrodes coated by a wax layer, on which in turn a layer of insulator synthetic resin is applied; the wax melts on heating, and a free space around the electrodes is produced by absorption into the synthetic resin layer. The synthetic resin material must therefore be very exactly matched with the used wax so that an absorption of the wax into the synthetic resin layer is insured.
German published application DOS No. 2,607,196 describes a piezoelectric filter having a lamella-like resonator which has two electrode pairs at two opposed main surfaces and where a damping material is applied on the outside of the electrode area. There the damping material consists of an elastic carrier material, which is mixed with a powdered material e.g., a metallic powder having a considerably higher specific weight. Though with this embodiment a satisfactory acoustic match between resonator and damping layer can be established so that reverberations can be avoided, no dip coating of the filter is possible because in that case the enveloping material would be directly applied to the electrodes so that any initially produced beneficial results would be negated. This applies with equal force to German Pat. No. 2,046,421.
German published application DOS No. 2,504,550 describes a piezoelectric filter in which a synthetic envelope contacts a lamella-like resonator without any interspacing. There an inter-spacing is avoided by a thin film made of paper, resin or synthetic interpolated between resonator and synthetic envelope.
German published application DOS No. 2,123,823 describes a ceramic filter in which a lamella-like resonator is solidly held in the space of an inner housing between two spring loaded plates.